How it works: intermittent fasting (IF) cycles between an eating window and a fasting window. Common patterns: 12:12 (beginner), 14:10, and 16:8 (the most-studied). After roughly 12–16 hours without food, insulin and liver glycogen drop and the body shifts toward fat oxidation and mild autophagy (cellular cleanup); most of these effects are gradual, not a switch that flips at an exact hour.
Evidence-backed benefits: improved insulin sensitivity, modest fat loss (mainly from the resulting calorie deficit), and simplified eating patterns. Effects on longevity and cognition are well-supported in animal studies but still preliminary in humans.
Optimizing your fast: water, black coffee/tea, and electrolytes (sodium/potassium/magnesium) don't break a fast and help with headaches or fatigue. Break your fast with protein + fiber rather than refined carbs to avoid a blood-sugar spike. 14–16h is a reasonable ceiling for most people — women often do better on the shorter end (12–14h) due to hormonal sensitivity to energy restriction.
Not recommended without medical guidance if you: are pregnant or breastfeeding, have a history of disordered eating, are underweight, or take insulin/blood-sugar medication.